Q: Are there any network boards available?
From Hardware
Digital made a board for the Rainbow to allow it's connection to a DECNet network. The board was called the Extended Communications Option. The board was connected to the front rightmost expansion slot (one of the expansion ports used by the hard disk controller). A panel was then removed from the rear of the Rainbow, allowing access to a DECNet port and a second serial port.
There were also rumors of a supposed EtherNet board. The board was developed by Univation, a third-party Rainbow accessory manufacturer. DEC had evaluated the board as an official solution, but found its power requirements to be too high.